Process for continuously anodizing aluminum

ABSTRACT

Aluminum is continuously anodized using direct current in an anodizing cell which is preceeded by a cathodic contact cell having an anode connected to a source of direct current. The aluminum already has an anodized oxide coating formed thereon before entering the contact cell and direct current is introduced into the already anodized aluminum in the contact cell for further anodizing the already anodized aluminum.

United States Patent [191 Fromson Nov. 18, 1975 PROCESS FOR CONTINUOUSLYANODIZING ALUMINUM [76] Inventor: Howard A. Fromson, l5 Rogues RidgeRoad, Weston, Conn. 06880 [22] Filed: Dec. 11, 1974 [21] Appl. No.2531,638

Related US. Application Data [63] Continuation-in-part of Ser. No.361,720, May 18,

1973, Pat. NO. 3,865,700.

[52] US. Cl. 204/28; 204/42; 204/211 [51] Int. Cl. C25D 7/00; C25D 5/00[58] Field of Search 204/28, 211, 267, 269,

[56] References Cited UNITED STATES PATENTS 2/1951 Odier 204/211 10/1969Cooke et a1. 204/28 3,510,410 5/1970 Rosenthal et al. 204/211 3,766,04310/1973 Herrmann et al 204/28 FOREIGN PATENTS OR APPLICATIONS 1,005,1913/1957 Germany 204/28 Primary Examiner-T. M. Tufariello Attorney, Agent,or FirmBurgess, Dinklage & Sprung [57] ABSTRACT Aluminum is continuouslyanodized using direct current in an anodizing cell which is preceeded bya cathodic contact cell having an anode connected to a source of directcurrent. The aluminum already has an anodized oxide coating formedthereon before entering the contact cell and direct current isintroduced into the already anodized aluminum in the contact cell 'forfurther anodizing the already anodized aluminum.

4 Claims, 1 Drawing Figure ANODIZE CONTACT CELL ANODIZING CELL US.Patent Nov. 18, 1975 PROCESS FOR CONTINUOUSLY ANODIZING ALUMINUM RELATEDAPPLICATIONS This application is a continuation-in-part of co-pendingapplication Ser. No. 361,720 filed May 18, 1973, now US. Pat. No.3,865,700.

BACKGROUND This invention relates to a process for continuouslyanodizing aluminum. The term aluminum is used herein to include aluminumbase alloys which, like pure aluminum, can be electrolytically anodizedto form oxide coatings. More particularly, this invention relates to atechnique for continuously anodizing coils or lengths of aluminum, suchas aluminum sheets, strips, wire, rods, shapes and the like (hereinaftercollectively referred to as aluminum web), by introducing anodizingdirect current via a cathodic contact cell into an aluminum web passingtherethrough which already has an oxide coating formed thereon.

The direct current is picked up in the cathodic cell by the alreadyanodized web and is transferred therealong in the direction of movementof the web to an anodizing cell where a further oxide coating is formed.

Aluminum and aluminum base alloys in sheet, strip and wire form havebeen continuously anodized by a number of techniques for many years.Such anodized products are used for electrical and decorative purposes,in the manufacture of household appliances, automotive trim, buildingmaterials, farm equipment, furniture, sporting goods, cans, containerclosures, lithographic plates, transformers, and in many other marketand product areas.

Two basic techniques are used to introduce current into a movingaluminum web. The first involves the use of a contact roll or bar andthe second is an electrochemical technique utilizing a cathodic contactcell.

The contact roll or bar technique suffers from many deficiencies. Forexample, the aluminum web must be dry to avoid electrolysis which, if itoccurs, dissolves the contact roller or bar anodically leaving pits inthe surface thereof. Another problem is arcing between the two surfacesas they become separated which is brought,about by the presence of edgeburrs or slivers of aluminum on the web surface itself. Arcing causespitting of the aluminum as well as pitting and oxidation of the contactmember itself.

When using the cathodic contact cell technique, one of the limits on howmuch current can be introduced into the web is the fact that all of thecurrent has to be Eintroduced into a single cross-sectional area of themoving web. This causes a surge of current into the unanodized web whichis unprotected by an oxide coating and tends to cause burning resultingin the formation of unsound oxide coatings. Up to now, the problem ofarcing in the use of a solid contact member and the problem of burningdue to a solid contact member, and the problem of burning due to a surgeof current in the 2 odizing through the oxide coating was thought to notbe possible.

The present invention now makes it possible to introduce anodizingcurrent into more than one cross-section of a moving aluminum web.

SU MMARY The present invention provides an improvement in a process forcontinuously electrolytically anodizing aluminum web. The directanodizing current is introduced into the aluminum web in a cathodiccontact cell, the web having an anodized oxide coating formed thereonbefore entering the contact cell.

Stated differently, the process of the invention for continuouslyelectrolytically anodizing aluminum web first involves continuouslyanodizing an aluminum Web using conventional techniques, preferably by acontact cell system, and then continuously passing the already anodizedaluminum web into a cathodic contact cell having an anode connected to asource of direct current, thereby introducing anodizing direct currentinto the already anodized aluminum web in the contact cell for furtheranodizing the already anodized aluminum web. Preferably, the contactcell has a second anode connected to a second source of direct currentand the unanodized aluminum web is continuously passed into a preceedinganodizing cell having a cathode connected to the second source of directcurrent. In this embodiment, anodizing direct current is introduced intotwo cross-sections of the web in the same contact cell for thepreceeding initial anodizing treatment and the succeeding furtheranodizing treatment.

DESCRIPTION OF THE DRAWING The present invention will be more fullyunderstood from the following description taken in conjunction with theaccompanying drawing which is a schematic flow diagram illustrating thefeatures of the invention.

DESCRIPTION The present invention makes it possible to introduceanodizing current into a moving aluminum web into more than onecross-section thereof where the web already has at least some protectiveoxide layer formed thereon. This can be accomplished by using a cathodiccontact cell, in which the aluminum web is cathodic, between twoanodizing cells in which the aluminum strip is anodic, or by utilizing amultiplicity of contactanodizing cells in which the aluminum web isalternatively negative or positive. When utilizing the preferredembodiment of the cathodic contact cell between two anodizing cells, thecurrent introduced into the contact cell will travel in both directionsthus effectively doubling the current carrying capacity of the movingaluminum web.

Referring now to the drawing, the process and apparatus for continuouslyelectrolytically anodizing aluminum web 12 includes conventionalanodizing using a contact roll/anodizing cell, a contact cell followedby an anodizing cell, or the reverse, an anodizing cell followed by acontact cell as described in my US. Pat. No. 3,865,700 where the web isanodized by the action of direct current introduced in the preceedingcontact cell. Next comes a contact cell indicated generally by referencenumeral 20 followed by an anodizing cell indicated generally byreference numeral 10. Each cell includes a suitable tank member 16 forcontaining an electrolyte 14. The anodizing cell 10 has a cathode l8therein connected to a source of direct current 24. The contact cell 20has an anode 22 therein which is connected to the same source of directcurrent 24. The aluminum web 12 with an already formed anodized coatingthereon continually passes through the contact cell 20 followed by theanodizing cell 10 with the aid of conventional guide rollers positionedas shown.

The anodizing direct current is introduced into the web 12 in thecontact cell 20. The web 12 has an anodized oxide coating already formedthereon before entering the contact cell 20 and the direct currentintroduced into the web 12 in the contact cell 20 produces furtheranodizing in cell 10.

In a preferred embodiment a second anodizing preceeds contact cell 20.The second cell contains a cathode connected to a second source ofdirect current. The contact cell contains a second anode which isconnected to the same second source of direct current. By utilizing acontact cell between two anodizing cells, the direct anodizing currentintroduced into the contact cell from the two separate sources of directcurrent travels in both directions thus effectively doubling the currentcarrying capacity of the moving aluminum web 12. Thus, anodizing currentis introduced into the web in the center contact cell 20 from onecurrent source which flows in a direction opposite to the direction ofmovement of the web into the preceding anodizing cell wherein an initialportion of the oxide coating is formed. The anodizing operation iscontinued in the second anodizing cell by anodizing current from asecond source which is picked up by the already anodized strip 12 in thecontact cell and is transmitted there along to the second anodizingcell. The web from the second anodizing cell with a given thickness ofoxide coating formed thereon and if desired, further anodizing can becarried out in accordance with the invention in a succeeding contactcell/anodizin cell set up as shown in the drawing and so on.

With respect to the basic embodiment shown in the drawing, the inventioncan be carried out by passing aluminum web 12 through two or moresuccessive pairs of contact cell 20 followed by anodizing cell 10.

As is well known in the art, the aluminum web 12 can be cleaned,de-greased or otherwise pretreated using conventional tehniques beforeanodizing and after anodizing, it can be sealed, dyed or otherwisepost-treated using known techniques. The web 12 is passed throughEXAMPLE 1 Aluminum samples were anodized in the sulfuric acidelectrolyte at constant electrolyte concentration and 5 temperatures. DCvoltage breakdown values were determined and are summarized in Table 1below:

TABLE 1 Time Time VOLTAGE CURRENT ANODIC CATHODlC BREAKDOWN (in amps)(sec.) (sec.) (volts DC) 25 6O 320 25 60 60 380 25 6O 30 380 25 30 29025 30 30 290 I5 290 20 20 20 220 The results of this Example indicatethat reversing the 20 polarity, or passing current through an oxidecoating, has no detrimental or adverse effect on the anodized oxidecoating. This is indicated by the DC voltage breakdown values which areidentical for samples which were subjected to cathodic time and forsamples 25 which were not so subjected to cathodic time.

EXAMPLE 2 Triple sets of aluminum samples were prepared and all wereanodized for the same period of time and at the same current density.Set number 1 was anodized only. Set number 2 was anodized and allowed tosit in the electrolyte while the polarity was reversed (samplesCathodic). Set number 3 was anodized and allowed to sit in the anodizingelectrolyte without applying any current. For all three sets theanodizing time, the time the polarity was reversed and the time theanodized samples were allowed to sit in the electrolyte was the same.

All of the samples were sealed in boiling water. The weight of the oxidecoating formed were determined by first weighing the sample and thenstripping off the oxide coating by soaking in a hot chromic-phosphoricacid solution and thereafter re-weighing the sample. The difference inweight divided by the total area of the sample gives the milligrams persquare inch of oxide coating on the original sample. The results aresummarized in Table 2.

TABLE 2 CURRENT ACTUAL VOLTAGE RE- TEMP. WEIGHT OF WEIGHT OF WEIGHT OF%WEIGHT LOSS VERSED TIME DENSITY CURRENT (volts) VOLTAGE (C) SET NO. 1SET NO. 2 SET NO. 3 BETWEEN SET (sec) (alft (amps) (volts) (mg/in)(mg/in) (mg/in) 1 and 2 I 40 15.5 16 2 30 5.57 5.45 5.2] 2 I50 40 15.513 2 40 5.09 4.62 4.67 9 I50 40 I l0 2 50 4.56 2.94 3.09 33 theanodizing operation of the invention using conventional windingandfeeding equipment.

The present invention will be more fully understood from the followingexamples which are not intended to limit or otherwise restrict theinvention in any way. In the examples, 4 X 8 inch sheets were employed.In all of the examples both sides of the aluminum sheets were anodizedand the electrolyte concentration was 230 grams per liter of aqueoussulfuric acid.

This example illustrates that there is very little difference in theweight of the oxide coating between samples where the polarity wasreversed and samples where the anodized samples were allowed to soak inthe anodizing electrolyte. This indicates that the loss in oxide weightis due primarily to solvent action of the electrolyte and is notattributable to the passage of current through the coating when thepolarity is reversed and the samples are cathodic. In evaluating thedata of Table 2 it should be pointed out that the solvent action of theanodizing electrolyte is increased as the temperature increases whichaccountsfor the greater loss in oxide weight at the tests runa-t highertemperatures.

EXAMPLE 3 l. The samplewas anodized at 40C. at a current density of 50a/ft for a specific length of time.

2. With the current density remaining at 50 a/ft the by the contact rollmethodor the standard contact cell method where all of the current isintroduced in one pass through a single cross-section of the web.

In an alternated embodiment the process of the invention for'furtheranodizing an-aluminum web already having an anodized oxide coatingthereon includes passing the already anodized aluminum web through acathodic contact cell under conditions of temperature, time andelectrolyte concentration (as demonstrated polarity was reversed makingthe sample cathodic 10 by the foregoing examples) such that the weightloss wh1le varylng In temperature and time. does not exceed50%.Preferablythe weight loss is not 3. The alummum sample was againanodized at 40C. greater than lO-l5% by weightjIn this manner the proata current dens1ty of 50 a/ft for a specific length of cess of theinvention increases the weight of the fintIme. I j ished anodizedcoating on the aluminum web. Weight Control samples were anodized at40C. at current loss can be described as the reduction in overall weightdensity of 50 a/ftf for a' period of time equalling the per unit of areaand is the result of the solvent effect of total time In steps 1 and 3.Step 2 was eliminated, the electrolyte on the anodized coating itself.In carry- All samples In this Example were sealed in hot water ing outthe process of the invention weight loss due to and the oxlde coatIngweights were determined as desolvent action is tolerated and is offsetby the greater scrlbed In Example 2. The data are summarized inTaincrease in the quantity, that is, thickness in weight of bles 3 and 4below: the finished anodized coating.

TABLE 3 TIME VOLTAGE TIME VOLTAGE TEMP TIME VOLTAGE TIME SAMPLE CONTROLWEIGHT STEP 1 STEP 1 STEP 2 STEP 2 STEP 2 STEP 3 STEP 3 STEP 1 COATINGCOATING LOSS OF (sec) (volts) (sec) (volts) (C) (sec) (volts) & 2 (sec)WEIGHT WEIGHT SAMPLE v (mglin (mg/in TABLE 4 7 TOTAL TEMP WEIGHT WEIGHT0F WE GHT LOSS AMPERE- STEP 2 OF SAMPLE CONTROL OF SAMPLE vs.

MIN

(C) (mg/in) (mg/in CONTROL Note: Ampere min. time anodized X currentdensity. co. 50 11/11 Anodizing Temperature 40C.

It is known that anodic oxide requires a forming voltage of somewherebetween 12 and 13 volts (of. Finishing Of Aluminum, Wernick and Pinner).The foregoing Examples demonstrate that when the polarity is reversedand the anodized aluminum samples are made the negative or cathodic poleof the cell, the anodized samples exhibit an unusual phenomenon and atthe same current density used to anodize, the voltage drops to between 1and 2 volts. Only minor heat is generated by resistance and there ispractically no weight loss as demonstrated herein. This unique propertymakes it possible to continue to introduce or feed current into thealuminum web at portions where the anodic oxide coating has already beenformed. By using this technique, it now becomes possible to continuouslyDC anodize aluminum webs without the limitations imposed The initialanodizing can be carried out as mentioned previously in an anodizingcell preceded by a contact roll or a contact cell for purposes ofintroducing the anodizing current into the web. The present invention ineffect starts in a contact cell with the web entering same having analready formed anodized coating thereon. The process of the inventioncan be duplicated as many times as desired using successive pairs ofcontact cells followed by an anodizing cell. It is important to notethat the initial contact cell utilized in the process of the inventionis not electrically connected to the initial anodizing step whichapplies an anodized oxide coating to the unanodized aluminum web.

It should also be noted that the preferred embodiment described aboveinvolving an anodizing cell followed by a second anodizing cell can becarried out with an anodizing section and a contact section contained inthe same tank so long as there is sufficient spacing between therespective electrodes for each sectron.

German Auslegeschrift 1,005,191 describes apparatus for forming etchedanodes for electrolytic condensers. The anode foil is anodized or formedwhile it passes through a forming bath. In the first portion of thebath, up-forming cathodes are used whose distance from the foildiminishes in the direction of passage as the coating is increasinglybuilt up, such that, at constant forming current density, the sum of theinverse voltage increas ing with the coating density and the voltagedrop which diminishes in the direction of passage, remains alwaysconstant in the forming electrolyte along the forming cathodes. Adown-forming section follows the up-forming section and serves todensify the coating produced in the up-forming section. Shieldingelectrodes are also utilized to de-gas the anode foil without anypotential drop in the electrolyte. The foil already has a positivecharge when it enters the forming bath. After the upforrning section thefoil is de-gased and the coating produced is densified. This publicationis thus limited in its forming voltage. Stated differently, thispublication is limited to the introduction of current through onecross-section of the foil before it enters the forming bath. Subsequentde-gasing and densifying sections do not introduce any further currentinto the foil. This is in contrast to the present invention which makesit possible to introduce anodizing current at a successive plurality ofcross-sections in the web, each with an anodized coating already formedthereon, as described in greater detail herein.

In the alternate embodiment wherein conditions of temperature, time andelectrolyte concentration are such that weight loss does not exceed 50%,when using aqueous sulfuric acid electrolyte, the concentration canrange from about to 350 grams per leader. Temperatures can range fromabout 20C to about 60C and anodizing times can run from about 20 secondsto about 5 minutes. Similar conditions will be employed with otheranodizing acids such as phosphoric acid, oxalic acid, chromic acid andthe like. In each instance anodizing time and temperature will depend onthe concentration and conductivity of the acqueous acid electrolyte.

What is claimed is:

1. In a process for continuously electrolytically anodizing aluminum,the improvement which comprises introducing anodizing direct currentinto said aluminum in a cathodic contact cell, said aluminum having ananodized oxide coating formed thereon before entering said cell.

2. Process for continuously electrolytically anodizing an alreadyanodized aluminum web comprises continuously passing said web through acathodic contact cell having therein an anode connected to a source ofdirect current, continuously passing said web from said contact cellinto an anodizing cell having therein a cathode connected to said sourceof direct current thereby introducing anodizing direct current into saidweb in said contact cell and further anodizing said web.

3. Process for further anodizing an anodized alumi- .num web whichcomprises passing said anodized web through a cathodic contact cell andan anodizing cell under conditions of temperature, time and electrolyteconcentration such that the weight loss of the anodized web leaving theanodizing cell does not exceed 50% thereby increasing the weight of theanodized coating on said web.

4. Process of claim 3 wherein the weight loss is from about 10 to about15%.

Disclaimer 3,920,525.H0w0ml A. Fmmson, Weston, Conn. PROCESS FORCONTINU- OUSLY ANODIZING ALUMINUM. Patent dated Nov. 18, 1975.Disclaimer filed J an. 24, 1977 by the inventor.

Hereby enters this disclaimer to claim 1 of said patent.

[Oyficz'al Gazette Mamh 8, 1.977.]

Dedication 3,920,525.Howm'd A. Fromson, Weston, Conn. PROCESS FORCONTIN- UOUSLY AN ODIZIN G ALUMINUM. Patent dated Nov. 18, 1975.Dedication filed Oct. 29, 1979, by the inventor.

Hereby dedicates to the Public the entire remaining term of said patent.

[Ofiaz'al Gazette January 22,1980.]

1. IN A PROCESS FOR CONTINUOUSLY ELECTROLYTICALLY ANODIZING ALUMINUM,THE IMPROVEMENT WHICH COMPRISES INTRODUCING ANODIZING DIRECT CURRENTINTO SAID ALUMINUM IN A CATHODIC CONTACT CELL, SAID ALUMINUM HAVING ANANODIZED OXIDE COATING FORMED THEREON BEFORE ENTERING SAID CELL. 2.Process for continuously electrolytically anodizing an already anodizedaluminum web comprises continuously passing said web through a cathodiccontact cell having therein an anode connected to a source of directcurrent, continuously passing said web from said contact cell into ananodizing cell having therein a cathode connected to said source ofdirect current thereby introducing anodizing direct current into saidweb in said contact cell and further anodizing said web.
 3. Process forfurther anodizing an anodized aluminum web which comprises passing saidanodized web through a cathodic contact cell and an anodizing cell underconditions of temperature, time and electrolyte concentration such thatthe weight loss of the anodized web leaving the anodizing cell does notexceed 50% thereby increasing the weight of the anodized coating on saidweb.
 4. Process of claim 3 wherein the weight loss is from about 10 toabout 15%.